Summary

In this lesson, students will use models to explore structural isomers, and create explanations for the impact of structure on intermolecular forces (London dispersion) and physical properties (boiling point).

Grade Level

High school

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Analyzing and Interpreting Data

AP Chemistry Curriculum Framework

This lesson supports the following learning objectives:

• Unit 3: Intermolecular Forces and Properties
  • Topic 3.1: Intermolecular Forces
    • SAP-5.A: Explain the relationship between the chemical structures of molecules and the relative strength of their intermolecular forces when:
      • The molecules are of the same chemical species.
      • The molecules are of two different chemical species.
  • Topic 3.2: Properties of Solids
    • SAP-5.B: Explain the relationship among the macroscopic properties of a substance, the particulate-level structure of the substance, and the interactions between these particles.
  • Topic 3.9: Separation of Solutions and Mixtures Chromatography
    • SPQ-3.C: Explain the relationship between the solubility of ionic and molecular compounds in aqueous and nonaqueous solvents, and the intermolecular interactions between particles.

Objectives

By the end of this lesson, students should be able to

• Draw structural isomers for a specified molecular formula
• Relate physical structure to intermolecular forces and boiling points

Chemistry Topics

This lesson supports students’ understanding of

• Molecules & Bonding
• Structural isomers
• Intermolecular forces (IMF), particularly London dispersion forces
• Physical properties, particularly boiling points
• Organic Chemistry
• Chemical information literacy

Time

Teacher Preparation: <30 minutes

Lesson: 1-2 class periods (80 to 130 minutes)

• Engage: 10 minutes
• Explore: 30-40 minutes (Part 1), 40-60 minutes (Part 2)
• Explain: 20 minutes
• Elaborate: 30 minutes
• Evaluate: 1 hour (approximately, per class)

Materials

• Molecular model Kits, recommended:
  • Darling Models
    
• Molecular Structure references/databases, such as:
  • The Sigma-Aldrich catalog
    
  • The CRC handbook
  • PubChem Database
  • ChemSpider Chemical Structure Database
    

Safety

• No specific safety precautions need to be observed for this activity.

Teacher Notes

• This resource could be used as a post-AP Chemistry exam activity.
• This lesson can be included within units on bonding and molecular structure, states of matter, intermolecular forces, organic chemistry, or structural isomers. This is appropriate for Honors and AP Chemistry students.
• This can be used as a standalone lesson to introduce isomers, the effect of structure on physical properties, and organic chemistry. In addition, it can be used in connection with another Chemistry of Cars resource, “Lab: Fractional Distillation of Crude Oil” which is a laboratory experiment, that uses fractional distillation to separate a mixture of organic molecules.
• London dispersion forces or induced dipole intermolecular forces are a common source of misconceptions for students. As necessary, review the different types of IMFs. This is a useful background video for London dispersion forces (Bozeman Science).
  

Engage: Ask students about a recent trip to the gas station and the different grades of gasoline to introduce the concept of octane number, isooctane, and octane isomers. The teacher may want to to have a picture of a gas pump showing octane numbers.

Use the podcasts below as an introduction (some students may be distracted by the British pronunciations of methyl, garage, petrol in the second podcast):

• A Moment of Science: What Do Octane Numbers Mean?
• Chemistry World: Octane Podcast

Leading Question: Why might octane and isooctane—compounds with the same chemical formula (C₈H₁₈)—have different combustion properties in an engine?

Explore:

• Part 1: Students will begin by drawing different isomers. The isomer drawing activity can be assigned as homework before the in-class modeling exercises (Part 2).
  
• Part 2: Students explore differences in physical interactions through examination of molecular models of octane isomers. The modeling exercise can be adapted for group jigsaw by assigning one molecule per group, then re-forming groups to share information.

Part 1: This activity can be adapted to the skill and experience of the students in your class. For example use the following suggested variations in student handouts provided:

• Octanes for “experts”
• Hexanes for “intermediates”
• Pentanes for “novices”

Students may find it useful to share ideas for systematically drawing structural isomers. In addition, it may be beneficial to provide a short period of time for all students to pool their structures. For example, students could be called upon to contribute structures to the board.

The Compound Interest resource A Brief Guide to Types of Organic Chemistry Formulae is a useful introduction to different representations for organic molecules.

Answers for each of the isomer variations in Part 1 are as follows:

• Novices: There are 3 isomers of C₅H₁₂: pentane, 2,2-dimethylpropane, and 2-methylbutane.
• Intermediates: There are 5 isomers of C₆H₁₄:
• hexane, 2,2-dimethylbutane, 2,3-dimethylbutane, 3-methylpentane, and 2-methylpentane.
• Experts: Some students may have difficulty independently drawing all 18 isomers of C₈H₁₈, especially some of the ethyl isomers. It is not necessary to draw all 18 isomers before proceeding, but a complete list of all isomers can be provided to students.

Part 2: The skeletal formulas below are a simplistic way to show preferred orientations for each set of octane isomers. A space-filling 3-D image is provided for reference (3-D images can be modeled and rotated on www.chemspider.org). Student answers for the modeling exercise should be similar to the following, though the level of detail and 3-dimensionality may vary. If appropriate, students may prefer to take photos of the models.

Answers for Part 2 are as follows:

Though London dispersion forces are typically characterized as weak in most textbooks, they are not inconsequential as we can see in the ~25 °C spread in boiling points for the isomers. More surface area = increased IMF = higher boiling points.

Conceptual Questions:

• How many different ways can carbon atoms and hydrogen atoms be arranged to form an alkane (how many structural isomers of octane or hexane or pentane are there)?
• Do you expect the different isomers to have the same physical properties (such as solubility, boiling point)?
• How might different isomers be separated from each other using only physical properties (physical separation methods include distillation)?

Explain: There is a characteristic odor associated with gasoline that can be connected to the physical properties of the hydrocarbon molecules. Many of the hydrocarbons in gasoline are small and have relatively weak London dispersion forces (thus low-boiling) and present in vapor phase near gasoline pumps; we can inhale and smell them.

Focus questions:

• What is the smell when you pump gas? Why does gasoline—a liquid—smell so strongly? Does the smell seem stronger on hot days?
• What keeps molecules together to form liquids?
• What happens to molecules during boiling/evaporation (liquid to gas transition)?
• Would a low boiling liquid have strong intermolecular forces?

From the activity, students should observe that the linearity of n-octane maximizes contact of adjacent molecule’s surface area. In contrast, the branching in 2,4-dimethylhexane and 2,2,4-trimethylpentane reduces surface area and potential interaction.

Elaborate: Analysis questions are provided in Part 2 for students to connect structure to physical properties (both intermolecular forces and boiling point). Students will consider how physical properties such a boiling point can be used to separate chemical compounds, and learn about the technique of fractional distillation in the laboratory and industry through this video.

Evaluate: Students will draw structural isomers and answer analysis questions to demonstrate understanding of the key concepts. Students will create a brief procedure for fractional distillation to demonstrate the use of physical properties as a means to separate chemical compounds.

Student explanations should relate the greater surface area of n-octane to the greater intermolecular forces between molecules of n-octane and thus a higher boiling point when compared to either 2,5-dimethylhexane or 2,2,4-trimethylpentane. Similarly, 2,5-dimethylhexane has a greater surface area and IMF than 2,2,4-trimethylpentane. In a fractional distillation, the lowest boiling component (2,2,4-trimethylpentane) is collected first.